The present invention relates to a fuel cell system which is able to discontinue power generation of a fuel cell with efficient discharging of remaining water and a method for discontinuing the fuel cell.
A fuel cell of solid polymer type has stacked structure of some tens to some hundreds layers of unit cells. Each unit cell includes electrically conductive separators sandwiching a Membrane Electrode Assembly (MEA), which has a hydrogen electrode and an oxygen electrode that together interpose an electrolytic membrane. A fuel cell system having this type of fuel cell discharges water, which is produced by a chemical reaction between hydrogen and oxygen, into an external environment via a discharge port. When the fuel cell system works in a cold district, it has occurred that water remaining in the fuel cell system freezes after termination of power generation, which has created a fear that membrane structure of an MEA experiences damage. In order to provide protection against this, it has been practiced that when a request for termination of power generation is given, scavenging for removing residual water from a fuel cell system is carried out before power generation has not been completely discontinued.
Japanese Published Patent Application 2002-313395 discloses a technique related to a discharging apparatus which prevents freezing of water remaining in a fuel cell system. This apparatus has two passages which can be switched by a switching valve. One passage is connected to a fuel cell via a gas cooler and gas humidifier, and the other one is bypassed, being connected to the upstream of a cathode back pressure valve. In this way, when the switching valve is directed to the cathode back pressure valve, it is possible to scavenge water remaining in the vicinity of cathode back pressure valve where a minimum flow rate occurs. On the other hand, when the switching valve is switched to the other direction, it is possible to scavenge water remaining in the fuel cell.
The technique disclosed in the patent document described above has a drawback that it is not possible to efficiently discharge remaining water, which may possibly result in damage for the fuel cell system due to freezing of the water. This drawback is ascribed to the fact that scavenging is carried out with a gas of low temperature and high humidity, which has undergone the gas cooler and the gas humidifier. If the temperature of gas is low, it is difficult to discharge remaining water by evaporation. Furthermore, if the gas is highly humidified, its absorption rate decreases, by which the capacity of gas to absorb remaining water is represented.
Because the scavenging gas always passes through the gas cooler and gas humidifier, a pressure loss adversely occurs, so that it is difficult to discharge the remaining water with a sufficient pressure.